Permanent Downhole Gauges: A Key To Optimize Deepsea Production

Abstract

ABSTRACT This paper shows the Brazilian 14 year experience (from 1977 to 1991), concerning 45 permanent installations of downhole pressure gauges, 41 installed in subsea completions in the Campos Basin, offshore Brazil. Some Brazilian and worldwide statistics regarding permanent gauges installation performance are also presented along with failure and diagnosis analysis. Petrobras Research Center has used these statistics to design a simple and reliable subsea installation, encompassing the state of the art and introducing some innovations. Applying this new design, Brazil intends to perform 65 permanent installations in subseawells (40 of them in deepwater: over 400 m / 1,312 ft of water depth) in the Campos Basin during the 90s. six out -of these 65 installations have already been performed in 1990 and 1991, including 3 very deepwater wells in the Marlim field. Reservoir and well monitoring, using downhole pressure and temperature gauges, minimizes costs by minimizing lost production and lowers accident risks through the reduction of workover operations. It also helps the production optimization through early diagnosis of formation damage, wax deposition, leaks, shut-downs, etc. The Brazilian design for permanent installations is based on simplicity, high integration among pertinent areas (operator and service companies), and on stringent feasibility studies for well choice. This design is supposed to yield large savings and increase the worldwide Mean Time To Failure (MTTF) from the current 3 years to 6 years by the end of the 90s, reaching the expected Brazilian mean time between workovers in subsea wells. INTRODUCTION Worldwid, the topic of permanent downhole sensors 1S still polemic. During an initial phase (late 70s, early 80s), the large failure rate, probably due to the introduction of the technology and the utilization of a logging cable without any type of encapsulation, created a large opposition to the installation of such systems. The statistics have shown clearly that the major part of the failures occurred inside the well. During the second phase of development of the permanent gauges technology in the mid 80s, the performance of these systems improved, but not to the point of reversing the negative feeling created by the failures of the pioneer installations. Notably, one noticed during this period the introduction of hydro acoustical transmission systems, andother innovations such as the utilization of a downhole cable protected by a nitrile encapsulation. The present phase of development of permanentgauge systems, started in 1986/1987. It is characterized by a stream of modifications such as the introduction of stainlesssteel encased cables (which guarantees the pressure integrity for the electrical connections at both ends), coupled with the utilization of metallic protectors. A bigger importance is given to the management of the project with the aim of a better interfacing of each component of the system. MUltiplexed subsea control systems are also appearing. New service companies offer permanent gauge systems, specially in the North Sea.